MultiRange Generics Suggestion

Nobody has claimed this yet.

https://github.com/jackc/pgx/blob/6f2ce92356f35263258ef7baa4cde86a3e1c6255/pgtype/multirange.go#L406
Might be looking for the following arcane generics invocation (the idea being that you require one constraint on the S and then require the other in a generic interface that embeds a *S)!

```go
type constraint[T pgtype.RangeValuer] interface {
pgtype.RangeScanner
*T
}

type Multirange[S pgtype.RangeValuer, T constraint[S]] []T

func (r Multirange[S, T]) IsNull() bool {
return r == nil
}

func (r Multirange[S, T]) Len() int {
return len(r)
}

func (r Multirange[S, T]) Index(i int) any {
return r[i]
}

func (r Multirange[S, T]) IndexType() any {
var zero T
return zero
}

func (r *Multirange[S, T]) ScanNull() error {
*r = nil
return nil
}

func (r *Multirange[S, T]) SetLen(n int) error {
*r = make([]T, n)
return nil
}

func (r Multirange[S, T]) ScanIndex(i int) any {
return &r[i]
}

func (r Multirange[S, T]) ScanIndexType() any {
return new(T)
}

```

Start in pgtype/multirange.go around line 406 and compare the existing Multirange implementation with the proposed generic constraints and methods. Determine whether the suggestion can be integrated while preserving the current scanning behavior; done means the design is resolved and the resulting behavior is covered by the relevant tests.

Written by the indexing model from the issue text.